Device for optically measuring surface properties

ABSTRACT

An apparatus for optical measurement of surface properties wherein a light spot is produced by an illumination device and reflected light is measured by a multiplicity of optical sensors. In the evaluation, the individual signals of sensors, at least a multiplicity of signals of comparatively small groups of sensors is taken into account. Thus, an extrapolation of the detection angle region is possible.

The present invention relates to an apparatus for optical measurementsof surface properties.

This is to be understood as an apparatus comprising an illuminationdevice to produce a light spot on the surface of a measurement object.Light reflected by the surface is detected and evaluated. Hereto, alarge number of optical sensors is provided which are arranged in anadequate manner. They should cover a certain and not too small area ofreflection angles, i. e. should be adapted to detect light reflectedunder these reflection angles. Further, an evaluation device is providedto evaluate the signals produced by the sensors in order to determinethe required surface properties of the measurement object.

A known apparatus of this kind is described in DE 38 05 785 A1. Thisdocument proposes, in order to determine surface profiles of materialsurfaces, to evaluate reflection angles of the reflection of laser lighton a surface by means of an angular arrangement of light sensors centredon the surface and irradiation of a light spot on the angular centre onthe surface by a laser. The surface profiles shall be determined fromthe reflection angles by integration. Therein, the reflection angles aredetermined by a series connection of the sensors and an evaluation ofsignals derived from the ends of this series connection, whereby acentre of mass is produced in the reflection angle determination.

The present invention has the technical object to provide an apparatusof the above described type that is improved compared to the cited priorart.

According to the invention, an apparatus is provided in which thesignals of a multiplicity of groups of sensors being respectivelyadjacent to each other are supplied to the evaluation device in agroup-wise and separated manner.

Preferred embodiments are defined in the dependent claims.

The basic idea of the invention is that in the series connection ofsensors proposed in the cited prior art a disadvantageously lowflexibility in the evaluation of the signals is produced. Although thesolution proposed in the prior art has the large advantage that only thetwo signals produced at the ends of the series connection must beconsidered, from which, further a single signal representing therequired centre of mass can be derived in a very simple manner, bydividing the difference and sum. However, this solution is limited to alinear centre of mass production. Namely, the coupling between thesensor can not be chosen freely so that the conventional apparatus cannot be used for different advantageous applications.

In contrast thereto, according to the invention, a multiplicity ofsignals from the sensor arrangement is used wherein those sensors thatare respectively combined to a common signal are named “a group” in thisdescription. Preferably, in this invention each group consists ofexactly one sensor so that each single sensor signal is taken intoaccount. The invention can, however, also be used by producing a commonsignal from a not too large group of adjacent sensors so that finally amultiplicity of sensor signals is provided that still represents thespatial resolution of the complete arrangement.

This provides for a substantially increased flexibility because thecoupling between the sensors or between the groups can be freely chosen.Possible applications are widespread wherein the invention is notlimited to special ones of these advantageous applications. E. g.signals from the respective sensors or groups can be logarithmized for afurther processing whereby a substantially increased dynamic range isprovided. Further, contributions of different sensors can be weighteddifferently and geometrical properties of the arrangement or sensitivitydifferences and comparable circumstances can be compensated thereby.Further, a background correction is feasible in measuring the signals ofthe groups without operating the illumination device and defining themas a background pattern. During the proper measurement, this signalpattern can be subtracted from the actual signals. Finally, the centreof mass determination can be conducted also in a non-linear manner, e.g. by square weighting. These and other possibilities to be describedhereunder need not be applicable simultaneously in order to use theinvention. Instead, the advantage of the invention is the fundamentalflexibility of the group-wise signal evaluation.

Especially, the surface properties to be measured can be a surfaceprofile (or contour) or characteristic data for a surface roughness. Themeasurement object is preferably, but not necessarily, a planar materialrun. The illumination device my comprise practically, but notnecessarily, a semiconductor laser diode. The sensors are preferablyarranged in a plane intersecting the surface to be measured in theregion of the light spot. Also further sensors can be provided inpossible further planes. Within the plane, the sensors should bearranged relatively dense in order to provide a substantially gap-freedetection of the reflected light along the required detection anglerange. Therein, the resolution and density of the arrangement can beadapted to the scattering angle width to be expected of the reflectedlight.

In total, preferably at least 21 sensors should be provided that aredivided into at least 7 groups.

The illumination device, e. g. the laser diode, can be directed in sucha manner that the surface of the measurement object is illuminatedobliquely. Thereby, the reflected light in case of a mirror reflectionon a planar surface is also inclined against the surface normal and alsoagainst the illumination direction. The arrangement of the sensors,which are usually arranged around the direction of such a mirror reflex,interferes less with the illumination device, i. e. need not to beinterrupted thereby.

Between the sensors and the surface to be measured, lens systems can beprovided in order to converge the reflected light on the sensors or todecrease its divergence. Therein, both large single lenses andarrangements of a multiplicity of lenses are possible. Single lenses canbe torical and can be curved concavely over the surface with respect tothe surface.

A measurement of the surface according to the invention can be conductedlocally, e. g. with one or a certain number of stationary singlemeasurements per measurement object. Therein, the light spot can be thatlarge that the required surface properties are detected over asufficiently large surface area. It is, however, preferred that arelative movement is produced between the light spot and the surfaceduring the measurement, i. e. the properties of the surface are measuredalong a track or a material run. Especially, it can be concluded fromthe reflection angles along the track or run on the profile progressionof the surface along the track or run by means of integration oraddition, what has already been described in the cited prior art. Besidethe possibility already described in the prior art, to move themeasurement object during the measurement and to hold the devicestationary, also the device can be moved. It is, however, preferred tomeasure with a stationary device on a moving measurement object. Theinvention especially relates to a production monitoring in productiontrains and production lines in which the measurement object, usuallysubstantially a plane material run, is transported anyway. This movementcan then be used for a measurement of the profile progression. Therein,the term “production” includes working steps. A mill train or a coatingtrain thus is also a production line.

Alternatively or additionally, there is the possibility to adapt theillumination device in order to be able to move the light spot by theillumination device along the surface. E. g. a polygon mirror scannercan be used hereto. This light spot movement can be used instead of amovement of the device or the measurement object surface or superposedthereon. Especially, quasi two-dimensional surface regions can bescanned thereby in a line by line manner.

A special advantage of the invention is that a determination of thescattering angle width of the reflected light is now possible. E. g. itcan be concluded from this scattering angle width to a micro roughnessthat can not be detected by the above mentioned profile determination.Therein, the scattering angle width of the reflected light can be usedin dependency on the locality or in terms of a local averaging.

An averaging of the scattering angle width values of the reflected lightcan also be used in order to extrapolate at the border of the detectionangle region defined by the arrangement of sensors. Namely, if a part ofa somewhat broadened “reflection light lobe” (or cone) extends over theborder of the detection angle region defined by the sensors, the centreof mass determination according to the prior art would lead to wrongstatements with regard to the centre of mass of this reflection lightlobe. This is because a part of the reflection light lobe is not takeninto account in the determination. If, however, a determination of thebroadening of this reflection light lobe in a temporal average is givenaccording to the invention, the centre of mass (or an otherwise definedcentre) can still be concluded from the reflection light lobe in suchcases. Under certain circumstances this can be extended such that even acentre of mass outside of the detection angle region can be calculatedby extrapolation as long as a part of the reflection light lobe is stilldetected. The proposed temporal averaging can be conducted in differentways. E.g., in the simplest case it can be conducted independent fromthe reflection angles, assuming that the underlying material propertiesdo not correlate with the reflection angles. However, also an averagingover the appearing width in the vicinity of the borders of the detectionangle range can be conducted in order to better adapt this averaging tothe conditions of the extrapolation.

A further possibility provided by the invention is a maximumdetermination instead of a centre of mass determination. This is notpossible with the conventional apparatus, either, as mentioned above. Itcan deliver more realistic information than a centre of massdetermination especially in case of asymmetric reflection light lobes,on the one hand. On the other hand, it can help to increase theeffective detection angle region because the detection of the maximum isstill possible when a part of the reflection light lobe is outside ofthe detectable angle region.

In the following, a preferred embodiment of the invention will beexplained in detail, wherein the individual features comprised can alsobe relevant for the invention in other combinations. Further, it is tobe clarified that the invention also has the character of a method sothat the preceding and the following description are to be understoodboth in view of the disclosure of apparatus features as well as ofmethod features.

Namely:

FIG. 1 shows a schematic diagram of the optical arrangement of ameasuring apparatus according to the invention and

FIG. 2 shows a schematic diagram of the arrangement of an evaluationdevice in this measuring apparatus.

In FIG. 1, 1 designates a sheet metal run as a measuring object. Sheetmetal run 1 is shown in cross section, wherein the longitudinaldirection is perpendicular to the plane of the drawing. Sheet metal run1 is material conveyed in a mill train which has been treated with skinpath rolls in a preceding working step not shown. Skin path rolls areused for the last procedure during sheet metal production and produce anecessary roughness on the sheet metal surface for later deep-drawingsteps or lacquer processes. Therein, the skin path roll may have beentreated by shot blast (metal ball bombardment) or electrical dischargemachining roughening (EBT, EDT) or by a laser roughening process (LaserTex) and transfers its surface properties on sheet metal 1. Dependingfrom the surface of sheet metal 1 as such (cold rolled, Zn plated . . .) the reflection and scattering properties of the surface change by thetreatment with the skin path roll. The broadening angle and the angledistribution of light intensity are interesting parameters for a qualitycheck.

The surface of sheet metal 1 is illuminated with focus light 2 of alaser diode not shown, wherein the average irradiation direction oflaser light 2 is perpendicular on sheet metal 1 in the sectional diagramof FIG. 1 and is drawn as being representative for the focussed lightbundle of the laser and numerated with 2. Reference numerals 3, 4 and 5show different variations of a reflection light lobe. 3 is a somewhatbroadened mirror reflex, 4 is a strongly broadened mirror reflex and 5is a strongly inclined reflex, the broadening of which correspondssubstantially to that of mirror reflex 3. These examples shallillustrate that the reflected light can change in view of the reflectionangle (compare the difference between 3 and 5) and the broadening(compare the difference between 3 and 4). The reflection anglerepresents an inclination of the reflecting surface portion of sheetmetal 1, namely in the region of the light spot produced by laser light2. The broadening represents a micro roughness that can only becharacterized by the width of the reflection lobe in a summary manner.

Reflection lobes 3, 4 and 5 as shown are drawn in their outer parts suchthat their outer shape symbolizes a typical intensity distribution ofthe reflected light within the respective lobe. The form of lobes 3, 4and 5 emanating from the illuminated spot on sheet metal 1 shall onlysymbolize the different sizes of the scattering angle width. The form ofthe sidelines is to no importance (and only somewhat concave for reasonsof the drawing).

The reflected light is measured by an arrangement of a multiplicity ofsensors 6 that is a circular arc around the light spot of laser light 2on sheet metal 1, wherein practically sensors 6 could be provided in amuch larger number. For cost reasons only 21 sensors 6 are providedhere. Of course, also other numbers are realizable.

FIG. 1 does not show that laser light 2 irradiated by the illuminationdevice is inclined with its optical axis against a surface normal onsheet metal 1, namely in a plane perpendicular to the plane of thedrawing and containing both the surface normal and the irradiationdirection of laser light 2. The circular arc arrangement of sensors 6defines a plane further comprising the light spot on sheet metal 1 whichalso is inclined against the surface normal, namely by the same amountin the opposite direction. One could thus imagine that laser light 2produces the light spot in the plane of the drawing on sheet metal 1from obliquely backwards in the perspective of FIG. 1, wherein thereflected light (reflection lobe 3) is reflected obliquely forward andimpinges on the angular arrangement of sensors 2. Compare FIGS. 3 and 5of the cited prior art DE 38 05 785 A1.

A lens array of single lens 7 is arranged stream upward to the circulararc arrangement of sensors 6 and corresponds to the sensors in aone-to-one relation, wherein the reflected light 3, 4 and 5 is focussedwith the lens array on sensor 6. Lines show a lobe width respectivelycorresponding roughly to the width of a single lens 7 leading to a spotfocussing on a sensor. This lobe width corresponds to an ideal mirrorreflex on sheet metal 1. As soon as a broadening appears in thereflection, a light detection in at least two sensors is produced,independent from the reflection angle.

FIG. 2 shows schematically the arrangement of an evaluation device ofthe apparatus described so far. On the left side, dark squares 6symbolize sensors 6 already shown in FIG. 1, only a part of which isshown. Each single sensor 6 is allocated to an AD converter 6 convertingthe originally analogue output signals of the sensors into digitalsignals. The digital signals from AD converters 8 are memorized into afast intermediate memory 9 (FiFo, first in, first out), namely for eachsensor individually, from which they can be read out in the memorizedorder as shown by the arrows.

A center of mass can be determined from the digitalized andintermediately memorized single signals in a portion 10 of theevaluation device. Therein both a linear center of mass determinationand a weightened center of mass evaluation or a square center of massevaluation are feasible.

Further, the individual signals are processed in a portion 11 of theevaluation device such that a typical value for the width of thedifferent reflection lobes 3, 4 and 5 shown in FIG. 1 is produced. Thisreflection lobe width is averaged in a temporal sense in a portion 12.

By means of this temporal average from portion 12, a further portion 13of the evaluation device can correct, taking into account the reflectionlobe width, the center of mass determined in portion 10 if it is tooclose to the borders of the angle region detected by the arrangement ofsensors 6, shown in FIG. 1 at the left and right outer sides,. E.g.reflection light lobe 5 is so far outward that a substantial part of thereflected light is no longer detected by sensors 6. The center of massdetermination in portion 10 is thus erroneous. This can be corrected inportion 13.

As long as the underlying assumption applies that the reflection widthsdo not correlate too strongly with the reflection angles, i.e. the microroughness does not correlate with the profile progression on thesurface, the detection angle region of the arrangement of sensors 6 canbe extended substantially. Namely, while with the above-discussedconventional arrangement of the cited prior art, a precise detection isonly possible as long as the complete reflection light lobe is withinthe detection angle region, the apparatus according to the invention canprovide an at least approximated calculation, even if only a part of thereflection light lobe, possibly not even the center of mass itself, isstill within the detection angle range.

Finally, also with this apparatus as in the cited prior art, it iscalculated from the reflection angles (namely here according to thecorrected centers of mass) in an integrating manner backwards to aprofile progression of the surface of sheet metal 1. This is not shownin detail in the drawing but thoroughly explained in the cited priorart, the technical disclosure of which is explicitly incorporated herein this respect and in respect of all further technical overlaps. Theprofile progression results from a movement of sheet metal 1 in thelongitudinal direction perpendicularly to the plane of drawing of FIG. 1so that the light spot moves along the surface relatively to sheet metal1. Further, the apparatus sketched in FIG. 1 can be extended such thatthe light spot is moved on the surface of the sheet metal 1 in thehorizontal direction of FIG. 1 line by line, for which e.g. a polygonmirror scanner can be used as also shown in the cited prior art in FIG.4 and the corresponding description. These are variations which areobvious to the expert in view of the prior art so that a detaileddescription is not necessary.

The embodiment could additionally be improved in that the individualsignals of light sensors 6 are corrected after the intermediatememorizing in fast memory 9 and before the already explained processingin portions 10 and 11 in view of an interference light background notcorrelated to the substantial measurement. Here too, prior to themeasurement a typical, possibly temporarily averaged signal pattern canbe taken from sensors 6, the laser diode being out of operation. Thissignal pattern can simply be subtracted from the signal patterns withoperating laser diode in a correcting manner so that the completeapparatus can be used without special darkening or blacking out and withsubstantially unchanged interference light conditions.

1. An apparatus for optical measurement of surface properties of ameasuring object comprising an illumination device for illuminating saidmeasuring object with a light spot, a multiplicity of at least threeoptical sensors arranged such that they can detect light irradiated bysaid illumination device and reflected by said surface of said measuringobject, and an evaluation device for evaluating signals of said sensorsfor determining said surface properties of said measuring object,wherein said evaluation device is programmed to be supplied with and todigitize signals from a multiplicity of at least three groups ofrespectively adjacent sensors in a groupwise and separated manner, andwherein a relative movement can be produced between said light spotproduced by said illumination device on said surface of said measuringobject and said surface so that said light spot is moved on said surfacealong a track and said evaluation device is programmed to calculate aprofile progression of said surface along said track from reflectionangles appearing along said track, and wherein said evaluation device isprogrammed to calculate a center of mass of said reflection angles whenat least two sensors detect light due to a broadening of saidreflection.
 2. An apparatus according to claim 1, in which exactly onesensor is provided for each group.
 3. An apparatus according to claim 1,wherein said sensors are arranged substantially along a circular archaving a center on said surface of said measuring object.
 4. Anapparatus according to claim 1, in which said illumination device isadapted to illuminate said surface of said measuring object in a mannerinclined against a normal on said surface.
 5. An apparatus according toclaim 1, having at least one lens for bundling the light reflected bythe surface of said measuring object onto said sensors.
 6. An apparatusaccording to claim 1, in which said sensors are stationary, saidmeasuring object is a material run to be conveyed along a productionline, and said relative movement follows from said conveying of saidmaterial run.
 7. An apparatus according to claim 1, in which said lightspot produced by said illumination device can be moved relatively tosaid surface of said measuring object by said illumination device.
 8. Anapparatus according to claim 1, in which said evaluation device isadapted for an extrapolation at borders of a detection angle range ofsaid detection device.
 9. An apparatus according to claim 8, in whichsaid evaluation device is adapted to determine a temporary average ofthe scattering angle width of the light reflected by said surface ofsaid measuring object, in order to extrapolate at a border of saiddetection angle range of said detection device if only a part of thelight of said scattering angle width can be detected.
 10. An apparatusaccording to claim 8, in which said evaluation device is adapted todetermine a maximum intensity within said scattering angle width of thelight reflected by said surface of said measuring object.
 11. A methodfor optical measurement of surface properties of measuring objectcomprising steps of illuminating said measuring object with a lightspot, detect light reflected by said surface of said measuring object bymeans of a multiplicity of at least three optical sensors, andevaluating signals of said sensors to determine said surface propertiesof said measuring object, wherein signals from a multiplicity of atleast three groups of respectively adjacent sensors are used anddigitized in a groupwise and separated manner for said evaluation, andwherein a relative movement is produced between said light spot on saidsurface of said measuring object and said surface so that said lightspot is moved on said surface along a track and a profile progression ofsaid surface along said track is calculated from reflection anglesappearing along said track, and wherein a center of mass of saidreflection angles is calculated when at least two sensors detect lightdue to a broadening of said reflection.
 12. A method according to claim11, in which exactly one sensor is provided for each group.
 13. A methodaccording to claim 11, wherein said sensors are arranged substantiallyalong a circular arc having a center on said surface of said measuringobject.
 14. A method according to claim 11, in which said surface ofsaid measuring object is illuminated in a manner inclined against anormal on said surface.
 15. A method according to claim 11, wherein thelight reflected by the surface of said measuring object is bundled witha lens onto said sensors.
 16. A method according to claim 11, in whichsaid sensors are stationary, said measuring object is a material runconveyed along a production line, and said relative movement followsfrom said conveying of said material run.
 17. A method according toclaim 11, in which said light spot is moved relatively to said surfaceof said measuring object by an illumination device.
 18. A methodaccording to claim 11, in which an extrapolation at borders of adetection angle range is conducted.
 19. A method according to claim 18,in which a temporary average of the scattering angle width of the lightreflected by said surface of said measuring object is determined, inorder to extrapolate at a border of said detection angle range if only apart of the light of said scattering angle width can be detected.
 20. Amethod according to claim 18, in which a maximum is determined withinsaid scattering angle width of the light reflected by said surface ofsaid measuring object.